1. Field of the Invention
Embodiments of the invention generally relate to fiber optic cables for use in harsh environments such as down hole gas and oil well applications.
2. Background of the Related Art
With advancements in the area of fiber optic sensors for use in harsh environments, there is an increasing need for fiber optic cables compatible with the harsh environmental conditions present in down hole oil and gas well applications. For example, fiber optic cables utilized in down hole sensing applications must be able to operate reliably in conditions that may include temperatures in excess of 300 degrees Celsius, static pressures in excess of 20,000 pounds per square inch (psi), vibration, corrosive chemistry and the presence of high partial pressures of hydrogen. As the sensors utilized in down hole applications may be positioned at depths up to and exceeding 20,000 feet, the fiber optic cable coupled thereto must be designed to support the optical fiber contained therein without subjecting the optical fiber to the strain associated with the weight of a long fiber suspended in a vertical orientation within a well without disadvantageously effecting the fiber's optical performance.
FIG. 7 depicts one example of a conventional fiber optic cable 700 suitable for use in harsh environments such as down hole oil and gas well applications. A similarly suitable cable is described in U.S. Pat. No. 6,404,961, issued Jun. 11, 2002 to Bonja, et al., which is hereby incorporated by reference in its entirety. Suitable cables are also available from Weatherford, Inc., located in Houston, Tex. The fiber optic cable 700, shown in FIG. 7, includes a fiber in metal tube (FIMT) core 702 surrounded by an outer protective sleeve 704. The FIMT core 702 includes an inner tube 706 surrounding one or more optical fibers 708. Three optical fibers 708 are shown disposed within the inner tube 706 in the embodiment of FIG. 7. A filler material 710 is disposed in the inner tube 706 to fill the void spaces not occupied by the optical fibers 708. The filler material 710 may also include a hydrogen absorbing/scavenging material to minimize the effects of hydrogen on the optical performance of the fiber 708. At least one of the inner or outer surface of the inner tube 706 is coated or plated with a low hydrogen permeability material 716 to minimize hydrogen diffusion into the area which in the optical fibers 708 are disposed.
The outer protective sleeve 704 includes a buffer material 712 and an outer tube 714. The buffer material 712 provides a mechanical link between the inner tube 706 and the outer tube 714 to prevent the inner tube 706 from sliding within the outer tube 714. Additionally, the buffer material 712 keeps the inner tube 706 generally centered within the outer tube 714 and protects the inner tube 706 and coatings formed thereon from damage due to vibrating against the outer tube 714.
Although this cable design has shown itself to be a robust and reliable means for providing transmission of optical signals in harsh environments such as oil and gas wells, the cable is one of the higher cost contributors to the overall cost of down hole sensing systems. Additionally, as the diameter of the cable is typically about one-quarter inch, the length of cable that may be transported on a spool using conventional means is limited to about 20,000 feet of cable. Thus, in many down hole well applications, only a single sensor may be coupled to a length of cable coming off a single spool, as the residual length of cable on the spool is not long enough for another down hole application without splicing on an addition cable segment. As cost is primary advantage of conventional metal conductor sensing systems over optical systems, a more cost effective optic cable suitable for down hole oil well service is highly desirable.
Therefore, there is a need for an improved fiber optic cable for use in harsh environments.